MUNICH, Germany - Western societies are losing their souls to scientific rationality and frightening believers in the developing world who still fear God, Pope Benedict told an open-air mass in Germany.

Benedict, on the second day of a visit to his native Bavaria, said that spreading the word of Jesus Christ was more important than all the emergency and development aid that rich churches like those in Germany gave to poor countries.

He also stressed the role of faith in fighting Aids "by realistically facing its deeper causes," indirectly confirming the Church view that pre-marital abstinence and fidelity in marriage are the way to combat sexually transmitted diseases.

About 250,000 faithful, many of them families with children, gathered at the fairground for the mass. "I've been here since five o'clock in the morning," said Kerstin Gessert, 32, from Karlsruhe. "I think it's important that he has come."

Wearing green and white vestments, the Pope addressed the crowd from a platform covered by a white canopy.

Some in the crowd wore traditional Bavarian folk dress and sat down to picnics of sausages and bread after the service.

"Social issues and the Gospel are inseparable," said the Pope. "When we bring people only knowledge, ability, technical competence and tools, we bring them too little," he said, hammering away at his central concern that secularisation and materialism have replaced faith in Western thinking.

Later, Benedict, 79, who has hinted the visit to his home region could be his last, led vespers at the Cathedral Of Our Lady in Munich where he served as archbishop from 1977 to 1982.

The twin green onion domes make the cathedral one of the city's best-known landmarks. Completed in 1488, it is Bavaria's largest church and has served as the cathedral for the archbishops of Munich and Freising since 1821.

Inside, the Pope, clad in glittering vestments of green, silver and gold, smiled broadly as young girls in first communion dresses and with flowers in their hair greeted him.

On his six-day trip, Benedict will also visit his birthplace at Marktl am Inn, the shrine to the Virgin Mary at Altoetting and Regensburg, where he taught theology from 1969 to 1977.

Police said the bright yellow house where the Pope was born in Marktl am Inn and which he was due to visit on Monday, was splashed overnight with two bags of blue paint by vandals.

At the morning mass Benedict said that Western societies had become "hard of hearing" about God, saying: "There are too many other frequencies in our ears. What is said about God strikes us as pre-scientific, no longer suited for our age."

He contrasted this to a faith he still found in developing countries, where 70 per cent of the world's Catholics now live.

"People in Africa and Asia admire our scientific and technical prowess, but at the same time they are frightened by a form of rationality which totally excludes God from man's vision, as if this were the highest form of reason," he said.

They sensed a "contempt for God" in Western societies and "a cynicism that considers mockery of the sacred to be an exercise of freedom and hold up utility as the supreme moral criterion for the future of scientific research," he said.

He singled out the German Catholic church, one of the world's richest, as one that generously gives emergency and development aid but plays down the spreading of the Gospel. "Evangelisation itself should be foremost," he declared.